Alpha-2 macroglobulin (A2MG) of the rat becomes demonstrable in the serum under natural and artificial conditions involving tissue growth and/or destruction. It is not found in healthy, non-pregnant adult rats. A closely related material has been demonstrated in human and other mammalian blood sera. The substance has a molecular weight of 0.8 to 1 X 10 to the 6th power daltons and is probably a glycoprotein. It is proposed that pure A2MG, obtained by a process developed in this laboratory, and antiserum to A2MG, as well as appropriate control substances, be added to dissociated cells from adult and fetal rats and from neoplastic tissues in order to determine the effects of these materials on cellular adhesion properties and their modification by lectins. Previous work has given reasons to believe it likely that one or more of these materials will modify such adhesion.